Method for producing aqueous styrene-butadiene polymer dispersions III

ABSTRACT

A process for preparing an aqueous styrene-butadiene polymer dispersion by free-radical aqueous emulsion polymerization of a monomer mixture M containing from 40 to 80% by weight of styrene as monomer M1, from 20 to 60% by weight of butadiene as monomer M2, and from 0 to 40% by weight, based on 100% by weight of monomers, of ethylenically unsaturated comonomers M3 other than styrene and butadiene by a monomer feed technique in the presence of from 0.05 to 0.5% by weight, based on 100% by weight of monomers, of at least one hydrocarbon HC having 6 to 20 carbon atoms, which is selected from compounds which on abstraction of a hydrogen atom form a pentadienyl radical or a 1-phenylallyl radical and from α-methylstyrene dimer comprises including at least 30% of the hydrocarbon HC in the initial charge to the polymerization vessel and supplying the remainder of the hydrocarbon HC to the polymerization reaction in the course of that reaction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing an aqueousstyrene-butadiene polymer dispersion by free-radical aqueous emulsionpolymerization of a monomer mixture comprising styrene and butadiene bya monomer feed technique.

2. Description of the Background

Aqueous styrene-butadiene copolymer dispersions find diverseapplication, particularly as binders in coating compositions such asemulsion paints and colored paper coating slips, in barrier coatings, asa coating for the back of carpets, as an adhesive base material incarpet adhesives, in building adhesives, for modifying mortar, cement,and asphalt, for consolidating nonwovens, in sealants, in foam moldings,and as binders for leather dressing.

These dispersions are generally prepared by free-radical aqueousemulsion polymerization of monomer mixtures comprising styrene andbutadiene. In the course of these processes, chain transfer agents areoften used in order to prevent excessive crosslinking of the polymers,which can have an adverse effect on the performance properties of thedispersion. Such substances regulate the molecular weight of the polymerchains as they are forming, and are therefore also known as regulators.

The prior art proposes a very wide variety of different substances foruse as regulators. Of commercial significance among these are compoundscontaining thiol groups, especially alkyl mercaptans such as n-dodecyland tert-dodecyl mercaptan (see, for example, Ullmann's Encyclopedia ofIndustrial Chemistry, 5th ed. on CD-ROM, Synthetic Rubber 2.1.2). Thesesubstances are disadvantageous in a variety of respects, however; forexample, because of their unpleasant odor, they are difficult to handleboth before and during the polymerization. Another disadvantage is theireffect on the inherent odor of the dispersions. This odor cannot becompletely suppressed even by means of complex deodorization measures.

The prior art has variously proposed other regulators for the emulsioncopolymerization of styrene with butadiene. In DE 195 12 999, forinstance, regulators containing sulfur are used in combination withhydrocarbons such as α-methylstyrene dimer and terpinolene asregulators.

EP-A 407 059 discloses a process for the emulsion polymerization ofmonomer mixtures comprising styrene and butadiene, which uses mixturesof terpinolene in combination with other chain transfer agents.

It has now been found that terpinolene and other hydrocarbons which onabstraction of a hydrogen atom form a pentadienyl radical or a1-phenylallyl radical as chain transfer agents, and also α-methylstyrenedimer alone, can be used as regulators. However, the dispersionsobtained contain large amounts of organic compounds which are of lowvolatility and which in some cases cannot be polymerized. The amount ofvolatile hydrocarbons in the resulting dispersions, even followingchemical deodorization of the dispersion, is generally above 3000 ppmand frequently above even 10 000 ppm. By chemical deodorization theskilled worker understands a postpolymerization process which isinitiated by free radicals and carried out under forced polymerizationconditions (see, for example, DE-A 44 35 423, DE-A 44 19 518, DE-A 44 35422 and literature cited therein).

The volatile hydrocarbons are primarily a result of the hydrocarbonregulator and low molecular mass, unpolymerizable reaction products ofthe styrene and of the butadiene, such as ethylbenzene,4-vinylcyclohexene, 4-phenylcyclohexene, and also unpolymerizedmonomers, especially styrene (i.e., residual monomer content) andunpolymerizable impurities in the feedstocks. High residual monomercontents are encountered especially when the amount of styrene in themonomer mixture to be polymerized is 40% by weight or more, and becomeall the more serious at styrene contents above 45% by weight, especiallyabove 50% by weight, and in particular above 55% by weight. Althoughhigh levels of volatile constituents can sometimes be removed bysubsequent physical deodorization, the expenditure, not least theexpenditure in terms of time, and hence the costs, rise as the residualmonomer content goes up. In particular the hydrocarbons that are used asregulators, however, can no longer be removed by conventional methods.Since, moreover, physical deodorization may have adverse consequencesfor the quality of the dispersion, a low level of volatile organicimpurities prior to deodorization is desirable from the standpoint ofquality as well.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forpreparing aqueous styrene-butadiene polymer dispersions in the presenceof hydrocarbon chain transfer agents which on abstraction of a hydrogenatom form a pentadienyl radical or 1-phenylallyl radical, as regulators,in the course of which smaller amounts of volatile constituents areformed.

We have found that this object is achieved by conducting thepolymerization in accordance with a monomer feed technique in the courseof which at least 30% of the hydrocarbon HC is included in the initialcharge to the polymerization vessel and the remainder of the hydrocarbonHC is supplied to the polymerization reaction in the course of thatreaction.

The present invention accordingly provides a process for preparing anaqueous styrene-butadiene polymer dispersion by free-radical aqueousemulsion polymerization of a monomer mixture M comprising

-   -   from 40 to 80% by weight, preferably 50 to 79% by weight, in        particular 55 to 79% by weight, of styrene as monomer M1,    -   from 20 to 60% by weight, in particular 20 to 49% by weight,        especially 20 to 44% by weight, of butadiene as monomer M2, and    -   from 0 to 40% by weight, e.g., 1 to 40% by weight, and        especially 1 to 25% by weight, based in each case on 100% by        weight of monomers, of ethylenically unsaturated comonomers M3        other than styrene and butadiene.        by a monomer feed technique in the presence of from 0.05 to 0.5%        by weight, in particular from 0.1 to 0.4% by weight, based on        100% by weight of monomers, of at least one hydrocarbon HC        having from 6 to 20 carbon atoms, as chain transfer agent, which        is selected from compounds which on abstraction of a hydrogen        atom form a pentadienyl radical or 1-phenylallyl radical, and        α-methylstyrene dimer, which comprises including at least 30%,        preferably at least 50%, in particular at least 70%, and with        particular preference the entirety, of the hydrocarbon HC in the        initial charge to the polymerization vessel and supplying the        remainder of the hydrocarbon HC to the polymerization reaction        in the course of that reaction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the invention is conducted in accordance with a monomerfeed technique. By this it is meant that the majority, usually at least70%, preferably at least 80%, and in particular at least 90%, or theentirety, of the total monomers to be polymerized are supplied to thepolymerization reaction under polymerization conditions. Bypolymerization conditions, the skilled worker understands that thepolymerization reactor contains an amount of initiator which issufficient to initiate the polymerization reaction and the reactorcontents are at a temperature at which the initiator has a decompositionrate which is sufficient for initiating the polymerization. Therelationships between temperature and decomposition rate are well knownto the skilled worker for the common polymerization initiators, or maybe determined in the course of routine experiments.

Concentration figures in monomer feeds, here and below, unless otherwisespecified, relate to the instantaneous concentration of one component inthe feed at the point in time at which it is added. Data on monomerconcentrations in % by weight refer to the entirety of the monomerssupplied at the point in time in question or in the time interval inquestion. By contrast, gross indications relate to the entirety of acomponent which is added over the entire duration of a feed. Unlessotherwise specified, a reference to the monomer feed is to be understoodas a reference to the sum of all monomer feed streams.

In one preferred embodiment of the invention, at a point in time atwhich at least 70% of the monomers to be polymerized have been suppliedto the polymerization reaction, the concentration of butadiene in themonomer feed is raised for a period of at least 1% of the total feedtime by at least 10% by weight, preferably by at least 15% by weight,e.g., by from 10 to 40% by weight, and in particular by from 15 to 30%by weight, based on monomers in the feed.

In general, the time interval in which the monomer feed has an increasedbutadiene concentration is at least 1%, and in particular at least 2%,of the total duration of the monomer feed and will preferably not exceeda duration of 20%, in particular of 10%, and will, for example, amountto from 1 to 20%, in particular from 2 to 10%, of the total duration ofthe monomer feed.

The concentration of butadiene in the monomer feed is preferably raisedto at least 50% by weight, in particular to at least 55% by weight.Accordingly, the styrene concentration during this period will bepreferably not more than 50% by weight and with particular preferencenot more than 45% by weight.

The change in composition of the feed takes place preferably when atleast 75%, and in particular at least 80%, and preferably before 99%, inparticular before 95%, and with particular preference before 90%, of themonomers to be polymerized have been supplied to the polymerizationreaction.

The change in the composition in the monomer feed may take placecontinuously or in stages in one or more steps, e.g., in 2, 3, 4, 5 or 6steps, to an end value or within a limited time interval which endsbefore the end of the addition of monomer.

The change in the composition of the monomer feed may be controlled in avariety of ways. For example, butadiene and styrene can be supplied tothe polymerization reaction by way of separate monomer feed streams.Alternatively, a portion of one kind of monomer, e.g., a portion ofbutadiene, is supplied to the polymerization reaction by way of a feedstream which is separate from the remaining amount of the monomers. Bychanging the relative feed rate of the monomer feed streams it is thenpossible in a simple way to bring about a change in the grosscomposition of the monomer feed. Of course, the monomers M1 and M2 andalso, where appropriate, M3 can also be supplied to the polymerizationreaction by way of a common feed and the instantaneous composition ofthe feed can be preadjusted by means suitable mixing devices whichpermit continuous mixing of fluid streams. Static mixers areparticularly suitable here.

In one preferred embodiment A, toward the end of the addition of monomerthe supply rate of the styrene-containing monomer feed is reduced, withthe rate of butadiene supplied constant, preferably such that thefraction of styrene in the monomer feed at the point in time of the endof the addition of monomer is less than 40% by weight, in particularless than 20% by weight, and especially 0% by weight. The change ispreferably made when 80%, in particular from 90 to 99.5%, and withparticular preference from 95 to 99%, of the monomers have beensupplied. A particularly simple way of achieving this is by ending thesupply of styrene before supply of butadiene has been ended, inparticular when from 90 to 99.5% by weight, and with particularpreference from 95 to 99% by weight, of the total butadienepolymerization have been supplied.

Conversely, with the rate of styrene addition constant, it is possibletoward the end of the addition of monomer to raise the rate of butadienesupply to a final value or at least to do so within a limited timeinterval (embodiment B). Additionally, the two measures can be combinedwith one another. As far as the duration of the phase of raisedbutadiene supply rate is concerned, the remarks made above apply.

Particular preference is given, as a special form of embodiment B, to anembodiment B′ in which a monomer mixture comprising styrene andbutadiene, and monomers M3 if desired, in an approximately constantmonomer composition is supplied to the polymerization reaction asmonomer feed Mf1, the fraction of butadiene in the gross composition ofMf1 being reduced by from 0.5 to 20% by weight, based on the totalamount of butadiene in the monomer composition to be polymerized. Whenat least 70%, preferably from 75 to 99%, and in particular from 80 to95%, of the monomer feed Mf1 have been supplied to the polymerizationreaction, from 0.5 to 20% by weight, preferably from 1 to 10% by weight,and in particular from 2 to 5% by weight, of butadiene, based on thetotal amount of the total butadiene to be polymerized, are added as afeed Mf2 in parallel with the remainder of the monomer feed Mf1 to thepolymerization reaction. Feed Mf2 will preferably contain less than 5%by weight of non-butadiene monomers M2 and/or M3. In particular, feedMf2 contains butadiene as the sole monomer. Mf2 can be added beginningat the abovementioned point in time through to the end of thepolymerization reaction, or within a short interval. The total durationof feed Mf2 is preferably from 1 to 20%, and in particular from 2 to10%, of the total duration of Mf1. The feeds Mf1 and Mf2 are to beunderstood as mass flows. Mf1 and Mf2 can be introduced into thepolymerization reactor by way of separate inlets. It is likewisepossible to introduce the amounts of monomer corresponding to the massflows Mf1 and Mf2 into the reactor by means of a common feed line, usingappropriate mixing equipment.

The monomers may be added either in the form of a mixture of themonomers as such or else in the form of an aqueous emulsion of themonomers M1 to M3, the latter procedure generally being preferred. Inembodiment B′ the butadiene-rich feed Mf2 is frequently supplied to thepolymerization reaction as pure monomer or monomer mixture and the feedMf1 as an aqueous emulsion.

Where the monomers are supplied to the polymerization reaction as anaqueous emulsion, the monomer fraction is usually from 30 to 90% byweight, in particular from 40 to 80% by weight, of the total weight ofthe emulsion. In addition, the monomer emulsion generally includes atleast part, preferably at least 70% by weight, in particular at least80% by weight, or the entirety, of the surface-active compounds whichare normally required for an emulsion polymerization.

In accordance with the invention, the process takes place in thepresence of at least one hydrocarbon HC as polymerization regulator. Itis of course also possible to tolerate small amounts of other compoundswhich are known to act as polymerization regulators. These include, forexample, the abovementioned compounds containing thiol groups, e.g.,alkyl mercaptans, and also the compounds specified in EP-A 407 059 andDE-A 195 12 999. Their fraction would generally amount to less than 0.1%by weight of the monomers to be polymerized and will preferably notexceed a fraction of 50 parts by weight, preferably 20 parts by weight,based on 100 parts by weight of hydrocarbon HC employed.

Suitable hydrocarbons HC, beside α-methylstyrene dimer, are all thosecompounds which on abstraction of a hydrogen atom form a pentadienyl or1-phenylallyl radical. These are compounds containing

-   -   either a 1,4-pentadiene structure with one or two hydrogen atoms        on the C3 atom (structure A):

-   -   or a 1,3-pentadiene structure with one or two hydrogen atoms on        the C5 atom (structure B):

it being possible for one of the double bonds to be part of a phenylring. In structures A and B, the vertical lines indicate open valences,without making any statement on the stereochemistry of the double bonds.The open valences can be satisfied with hydrogen, an alkyl group or aphenyl group, or each 2 open valences may form a 5- or 6-memberedcarbocyclic ring. Valences on two carbon atoms connected to one anotherby a double bond may combine with the carbon atoms of the double bond torepresent a phenyl ring.

Examples of compounds of structure A are 1,4-dihydrobenzene,γ-terpinene, terpinolene, and norbornadiene. Examples of hydrocarbons ofstructure B are 1,3-cyclohexadiene, α-terpinene, and α-phellandrene. Theterm “hydrocarbon HC” also embraces hydrocarbon ketones such as α-iononeand hydrocarbon alcohols which eliminate water to form a structure A orB. Preferred hydrocarbon regulators are γ-terpinene, terpinolene, andα-methylstyrene dimer, especially terpinolene.

Suitable polymerization initiators include in principle all thosecompounds which are known to be suitable for initiating a free-radicalpolymerization, especially that of butadiene and styrene. Preference isgiven to those initiators which contain a peroxide group, such asorganic and inorganic peroxides and hydroperoxides. Particularpreference is given to hydrogen peroxide and the salts ofperoxodisulfuric acid, e.g., sodium peroxodisulfate. Also suitable areorganic hydroperoxides such as tert-butyl hydroperoxide and cumenehydroperoxide. In some cases it has been found suitable to use theaforementioned peroxides together with a reducing agent and/or a metalcompound which is able to change its valence state. Suitable reducingagents are ascorbic acid, hydroxymethanesulfinic acid, the bisulfiteadduct of acetone, sodium sulfite, and sodium hydrogen sulfite. Examplesof suitable metal compounds are the salts and water-soluble complexes ofiron; of vanadium or of copper. Very particular preference is given inthe process of the invention to using peroxodisulfates such as sodiumperoxodisulfate as polymerization initiators. Preferred initiators aresoluble in water.

The free-radical initiator (polymerization initiator) is normally usedin an amount from 0.2 to 5% by weight, in particular from 0.5 to 3% byweight, based on the monomers to be polymerized. The free-radicalinitiator is generally added at the rate at which it is consumed.Accordingly, it is usual to supply at least a portion or the entirety,preferably at least 50%, in particular at least 80%, of the initiator tothe polymerization reaction in the course of the polymerizationreaction, preferably in parallel with the addition of monomer. Inparticular, from 2 to 50% and with particular preference from 5 to 20%of the initiator are included in the initial charge to the reactionvessel, this initial charge is heated to the desired polymerizationtemperature, and the remaining amount of initiator is supplied to thepolymerization reaction in parallel with the addition of monomer at aconstant or variable feed rate, e.g., a climbing or falling feed rate,or at the rate at which it is consumed.

The initiator can be used either per se or as a dispersion or solutionin an appropriate solvent. Suitable solvents are in principle allcustomary solvents which are able to dissolve the initiator. Preferenceis given to water and water-miscible organic solvents, e.g., C₁-C₄alcohols, or mixtures thereof with water. In particular, the initiatoris added in the form of an aqueous solution. With preference, theaddition of initiator is ended together with the end of the addition ofmonomer or no later than 1 h, in particular no later than half an hour,after the end of the addition of monomer.

The polymerization temperature naturally depends on the decompositioncharacteristics of the polymerization initiator and is preferably atleast 60° C., in particular at least 70° C., with particular preferenceat least 80° C., and with very particular preference at least 90° C.Normally, a polymerization temperature of 120° C. and preferably 110° C.will not be exceeded, so as to avoid complex pressure apparatus. With anappropriate choice of reaction vessel, however, it is also possible toemploy temperatures above these levels. In the case of what is known ascold operation, i.e., when using redox initiator systems, it is evenpossible to carry out polymerization at relatively low temperatures,such as from 10° C. upward, for example.

For reducing the level of residual volatiles it has proven advantageousto supply the monomers as fast as possible to the polymerizationreaction. The monomers to be polymerized are preferably supplied to thepolymerization reaction over the course of not more than 5 hours, inparticular within a period of from 1 to 4 hours, with particularpreference within a period of from 2 to 4 hours.

Furthermore, it has proven advantageous to subject the reaction mixtureto intensive mixing during the polymerization. Intensive mixing can beachieved, for example, by using special stirrers in conjunction withhigh stirring speeds, by combining stirrers with stators or by rapidcirculation, e.g., pump circulation, of the reaction mixture via abypass, it being possible for the bypass in turn to be equipped withdevices for generating shear forces, e.g., solid internals such asshearing plates or perforated plates. By special stirrers are meantthose stirrers which generate not only a tangential flow component butalso an axial flow field. Stirrers of this kind are described, forexample, in DE-A 197 11 022. Multistage stirrers are particularlypreferred. Examples of special stirrers for producing tangential andaxial flow components are cross-arm stirrers, MIG® and INTERMIG®stirrers (multistage impulse countercurrent stirrers and interferencemultistage impulse countercurrent stirrers from EKATO), axial-flowturbine stirrers, it being possible for the aforementioned stirrers tobe single-stage or multistage in construction and to be combined withconventional stirrers, and, additionally, helical stirrers, preferablyin close-clearance versions, coaxial stirrers, comprising ananchor-shaped close-clearance stirrer and a single-stage or multistagehigh-speed central stirrer, and also multiple-blade stirrers. Alsosuitable are types of stirrer described in DE-C1 4421949, JP-A 292002,and WO 93/22350.

Furthermore, it has proven advantageous to conduct the process of theinvention such that the density of the polymer particles in the finisheddispersion does not fall below a level of about 5×10¹⁶ particles per kgof dispersion and is situated in particular in the range from 10¹⁷ to3×10¹⁹ particles/kg of dispersion. The particle density is dependent, ofcourse, on the average diameter of the polymer particles in thedispersion. The average diameter of the polymer particles willpreferably be below 300 nm and more preferably will be situated withinthe range from 50 to 200 nm. The average particle diameter is defined,as is conventional, as the weight average of the particle size asdetermined by means of an analytical ultracentrifuge in accordance withthe method of W. Scholtan and H. Lange, Kolloid-Z. und Z. Polymere 250(1972) pages 782 to 796, (see also W. Mächtle in “AnalyticalUltracentrifugation in Biochemistry and Polymer Science”, S. E. Hardinget al (ed.), Cambridge: Royal Society of Chemistry, 1992, pp. 147-175).The ultracentrifuge measurement yields the integral mass distribution ofthe particle diameter of a sample. From this it is possible to inferwhat percentage by weight of the particles has a diameter equal to orless than a certain size. Similarly, the weight-average particlediameter can also be determined by dynamic or quasielastic laser lightscattering (see H. Wiese in D. Distler (ed.) “WässrigePolymerdispersionen”, Wiley-VCH, Weinheim 1999, p. 40 ff. and literaturecited therein). Measures for adjusting the particle density and theaverage particle diameter of aqueous polymer dispersions are known tothe skilled worker, for example, from N. Dezelic, J. J. Petres, G.Dezelic, Kolloid-Z. u. Z. Polymere 242 (1970), pp. 1142-1150. It can becontrolled both through the amount of surface-active substances andthrough the use of seed polymers, known as seed latices, with highemulsifier concentrations and/or high concentrations of seed polymerparticles generally producing low particle diameters.

In general it proves advantageous to conduct the emulsion polymerizationin the presence of one or more very finely divided polymers in the formof aqueous latices (known as seed latices). It is preferred to use from0.1 to 5% by weight, and in particular from 0.2 to 3% by weight, of atleast one seed latex (solids content of the seed latex, based on totalmonomer amount). Some or all of the seed latex may be supplied to thepolymerization reaction together with the monomers. Preferably, however,the process takes place with seed latex included in the initial charge(initial-charge seed). The latex generally has a weight-average particlesize of from 10 to 200 nm, preferably from 20 to 100 nm, and inparticular from 20 to 50 nm. Examples of its constituent monomersinclude styrene, methyl methacrylate, n-butyl acrylate, and mixturesthereof, it being possible as well for the seed latex to contain incopolymerized form, to a minor extent, ethylenically unsaturatedcarboxylic acids, e.g., acrylic acid and/or methacrylic acid and/ortheir amides, preferably at less than 10% by weight, based on the totalweight of the polymer particles in the seed latex.

When using a seed latex a procedure often followed is to include all orsome of the seed latex, preferably at least 80% of it, in the initialcharge to the polymerization vessel, to add some of the initiator,preferably in the fractions indicated above, and, where appropriate,some of the monomers to be polymerized, and to heat the mixture to thedesired polymerization temperature. It is of course also possible tointroduce the initiator and the seed latex in the opposite order. Themonomers are preferably not added until polymerization conditionsprevail. As well as the initiator and the seed latex, the initial chargenormally includes water and, where appropriate, a portion of thesurface-active compounds.

In general, a pH of 9 will not be exceeded during the polymerization.The pH is controlled in a simple way by adding a neutralizing agent inthe course of the polymerization reaction. Suitable examples includebases such as alkali metal hydroxide, carbonate or hydrogen carbonate,if the pH rises during the polymerization. This is the case, forexample, when using peroxodisulfates as polymerization initiators.

The polymerization reaction is frequently followed by apostpolymerization for the purpose of reducing the amount of unreactedmonomers in the dispersion (referred to as residual monomers). Thispostpolymerization is often also termed a chemical deodorization.Chemical deodorization generally takes place by free-radicalpostpolymerization, especially under the action of redox initiatorsystems, such as are listed, for example, in DE-A 44 35 423, DE-A 44 19518, and DE-A 44 35 422. The postpolymerization is preferably conductedwith a redox initiator system composed of at least one organic peroxideand one reducing agent, preferably an inorganic sulfite or the salt ofan α-hydroxy sulfone or an α-hydroxy sulfinic acid (adduct of hydrogensulfite with carbonyl compound). The amounts of initiator for thepostpolymerization are situated generally within a range of from 0.1 to5% by weight, preferably in the range from 0.2 to 3% by weight, and inparticular in the range from 0.3 to 2% by weight, based on the totalmonomers polymerized. In the case of initiator systems composed of aplurality of components, such as the redox initiator systems, theamounts relate to the total amount of these components. The chemicaldeodorization is conducted preferably at temperatures in the range from60 to 100° C. and in particular in the range from 70 to 95° C. Theamount of initiator used for chemical deodorization may be added to thedispersion in one portion or continuously over a prolonged period at aconstant or varying—e.g., increasing—feed rate. The duration of additionis then generally in the range from 10 minutes to 5 hours, and inparticular in the range from 30 minutes to 4 hours. The total durationof the chemical postpolymerization is generally in the range from 15minutes to 5 hours, and preferably in the range from 30 minutes to 4hours.

The preparation of aqueous styrene-butadiene copolymer dispersions usingterpinolene by the process of the invention gives dispersions having amuch lower residual monomer content than in the prior art processes forpreparing comparable dispersions. Following the chemical deodorizationwhich is commonly carried out, dispersions can be obtained whosevolatile organic compounds content is well below 10 000 ppm, preferablybelow 3000 ppm, in particular below 2500 ppm, and especially below 2000ppm.

Of course, the level of volatile organic constituents can be reducedstill further by known methods. This can be achieved, conventionally, byphysical means, by distillative removal (especially by steamdistillation) or by stripping with an inert gas, or by adsorption (seeR. Racz, Macromol. Symp. 155, 2000, pp. 171-180). Following thepolymerization reaction it is preferred first of all to carry out achemical deodorization and after that a physical deodorization. Bothmeasures may also be carried out simultaneously.

As regards the monomers M3 there are in principle no restrictions in theprocess of the invention. Rather, the nature and amount of the monomersM3 is guided primarily by the intended use. Examples of suitablemonomers M3 are:

-   -   monoethylenically unsaturated, acid-functional monomers such as        monocarboxylic and dicarboxylic acids having from 3 to 10 carbon        atoms such as acrylic acid, methacrylic acid, crotonic acid,        acrylamidoglycolic acid, vinylacetic acid, maleic acid, itaconic        acid, and the monoesters of maleic acid with C₁-C₄ alkanols,        ethylenically unsaturated sulfonic acids such as vinylsulfonic        acid, allylsulfonic acid, styrenesulfonic acid,        2-acrylamidomethylpropanesulfonic acid, and ethylenically        unsaturated phosphonic acids, such as vinylphosphonic acid,        allylphosphonic acid, styrenephosphonic acid and        2-acrylamido-2-methylpropanephosphonic acid, and their        water-soluble salts, their alkali metal salts, for example;        preferably acrylic acid and methacrylic acid. Monomers of this        kind may be present among the monomers M in an amount of up to        10% by weight, e.g. from 0.1 to 10% by weight, preferably from        0.1 to 4% by weight;    -   amides of monoethylenically unsaturated carboxylic acids, such        as acrylamide and methacrylamide, and also the        N-(hydroxy-C₁-C₄-alkyl)amides, preferably the N-methylolamides        of ethylenically unsaturated carboxylic acids, such as        N-methylolacrylamide and N-methylolmethacrylamide. Monomers of        this kind may be present among the monomers M in an amount of up        to 10% by weight, e.g. from 0.1 to 10% by weight, preferably        from 0.1 to 4% by weight;    -   hydroxyalkyl esters of monoethylenically unsaturated carboxylic        acids, especially hydroxyethyl, hydroxypropyl, and hydroxybutyl        esters, e.g. hydroxyethyl acrylate, hydroxypropyl acrylate,        hydroxyethyl methacrylate, and hydroxypropyl methacrylate.        Monomers of this kind may be present among the monomers M in an        amount of up to 10% by weight, e.g., from 0.1 to 10% by weight,        preferably from 0.5 to 5% by weight;    -   ethylenically unsaturated nitriles having preferably from 3 to        10 carbon atoms, such as acrylonitrile and methacrylonitrile.        Monomers of this kind may be present among the monomers M in an        amount of up to 30% by weight, e.g., from 1 to 30% by weight,        preferably from 5 to 20% by weight;    -   reactive monomers: the reactive monomers include those which        have a reactive functionality that is suitable for crosslinking.        In addition to the abovementioned ethylenically unsaturated        carboxylic acids, their N-alkylolamides, and hydroxyalkyl        esters, these include monomers which contain a carbonyl group or        epoxy group, examples being N-diacetoneacrylamide,        N-diacetonemethacrylamide, acetylacetoxyethyl acrylate, and        acetylacetoxyethyl methacrylate, glycidyl acrylate, and glycidyl        methacrylate. Monomers of this kind may be present among the        monomers M in an amount of up to 10% by weight, e.g., from 0.5        to 10% by weight; and    -   crosslinking monomers: the crosslinking monomers include those        which have at least two nonconjugated ethylenically unsaturated        bonds, e.g., the di- and tri-acrylates and -methacrylates of        difunctional and trifunctional alcohols, e.g., ethylene glycol        diacrylate, diethylene glycol diacrylate, triethylene glycol        diacrylate, butanediol diacrylate, hexanediol diacrylate,        trimethylolpropane triacrylate, and tripropylene glycol        diacrylate. Monomers of this kind may be present among the        monomers M in an amount of up to 2% by weight, preferably not        more than 1% by weight, e.g., from 0.01 to 2% by weight,        preferably from 0.01 to 1% by weight. In one preferred        embodiment the monomers M contain no crosslinking monomer.

Preferred monomers (monomers M3′) are the monoethylenically unsaturatedmonocarboxylic and dicarboxylic acids having from 3 to 10 carbon atoms,their amides, their C₂-C₄ hydroxyalkyl esters, their N-(hydroxy-C₁-C₄alkyl)amides and the above-mentioned ethylenically unsaturated nitriles.Particularly preferred comonomers are the monoethylenically unsaturatedmonocarboxylic and dicarboxylic acids, especially acrylic acid,methacrylic acid, and itaconic acid.

In one particularly preferred embodiment of the process of the inventionthe mixture of monomers M to be polymerized comprises

-   -   from 55 to 70% by weight of styrene,    -   from 29 to 44% by weight of butadiene, and    -   from 1 to 10% by weight of at lest one monomer M3, preferably at        least one monomer M3′, and in particular an ethylenically        unsaturated monocarboxylic or dicarboxylic acid.

In another preferred embodiment of this process, some of the styrene,preferably from 5 to 20% by weight, based on the total monomer amount,is replaced by acrylonitrile and/or methacrylonitrile. In this preferredembodiment the mixture to be polymerized comprises, for example,

-   -   from 30 to 65% by weight of styrene,    -   from 29 to 44% by weight of butadiene,    -   from 5 to 25% by weight of acrylonitrile and/or        methacrylonitrile, and    -   from 1 to 10% by weight of an ethylenically unsaturated        monocarboxylic or dicarboxylic acid.

In the light of the use of the polymers prepared by the process of theinvention as binders in coating compositions, e.g., in colored papercoating slips or in paints, it has proven advantageous if the polymerresulting from the polymerization has a glass transition temperature inthe range from −20 to +50° C. and preferably in the range from 0 to 30°C. The glass transition temperature here is the midpoint temperaturewhich can be determined in accordance with ASTM 3418-82 by means of DSC.

The glass transition temperature can be controlled in a known waythrough the monomer mixture M employed.

According to Fox (T. G. Fox, Bull. Am. Phys. Soc. (Ser. II) 1, 123[1956] and Ullmanns Encyklopädie der Technischen Chemie, Weinheim(1980), pp. 17, 18) the glass transition temperature of copolymers athigh molar masses is given in good approximation by

$\frac{1}{T_{g}} = {\frac{X^{1}}{T_{g^{1}}} + \frac{X^{2}}{T_{g^{2}}} + \mspace{14mu}{\ldots\mspace{14mu}\frac{X^{n}}{T_{g^{n}}}}}$where X¹, X², . . . , X^(n) are the mass fractions of the monomers 1, 2,. . . , n and T_(g) ¹, T_(g) ², . . . , T_(g) ^(n) are the glasstransition temperatures of the polymers constructed in each case fromonly one of the monomers 1, 2, . . . , n, in degrees Kelvin. Thesetemperatures are known, for example, from Ullmann's Encyclopedia ofIndustrial Chemistry, VCH, Weinheim, Vol. A 21 (1992) p. 169 or from J.Brandrup, E. H. Immergut, Polymer Handbook 3^(rd) ed., J. Wiley, NewYork 1989. Accordingly, polystyrene possesses a T_(g) of 380 K andpolybutadiene a T_(g) of 171 K or 166 K.

The examples which follow are intended to illustrate the invention butwithout restricting it.

The particle size of the polymer was determined by light scattering inaccordance with ISO 13321 using a Malvern autosizer 2C on samples with aconcentration of 0.01% by weight. The light transmittance was determinedon samples with a concentration of 0.01% by weight at a cuvette lengthof 2.5 cm against pure water as reference. The glass transitiontemperature was determined by means of DSC by the midpoint method.

Residual volatile fractions were determined by gas-chromatographicanalysis.

EXAMPLE 1

A polymerization vessel was charged with 360 g of water, 91 g of a 33%by weight polymer seed (polystyrene latex, d₅₀ 30 nm), 5.0 g ofterpinolene and 10% of the initiator solution (feed stream 2) and thisinitial charge was heated to 90° C.

Then the remainder of the monomer emulsion and the remainder of theinitiator solution were added to the polymerization vessel by way of twoseparate feeds, beginning simultaneously, over the course of 2.5 hours,during which the temperature was maintained. 2 hours after the beginningof feed stream 1, 30 g of butadiene were introduced into the reactionvessel over the course of 5 minutes. After the end of the addition ofmonomer, the mixture was cooled to 85° C. and then an aqueous solutionof 8.5 g of tert-butyl hydroperoxide in 90 g of water, and also asolution of 3.9 g of acetone and 15.7 g of a 40% strength by weightaqueous sodium disulfite solution in 84 g of water were added, beginningsimultaneously, over the course of 2 hours, during which the temperaturewas maintained. Thereafter, 24.6 g of 25% strength by weight sodiumhydroxide solution were added and the batch was cooled to roomtemperature.

Feed Stream 1:

540.0 g deionized water  36.6 g emulsifier solution 800.0 g styrene640.0 g butadiene  45.0 g acrylic acid  12.0 g 25% strength by weightaqueous sodium hydroxideFeed Stream 2:

-   -   15 g sodium peroxodisulfate in 210 g water

Emulsifier solution: mixture of 3 parts by weight of an aqueous 45%strength by weight solution of the sodium salt of disulfonatedmonododecyldiphenyl ether (DOWFAX 2A1, Dow Chemical) and 7 parts byweight of aqueous 15% strength by weight sodium dodecyl sulfatesolution.

The solids content of the dispersion was about 50% by weight. The lighttransmittance was 72.5%. The weight-average particle size d₅₀ was 120nm. The pH was 5.6 and the glass transition temperature T_(g) was 5° C.

EXAMPLE 2

A polymerization vessel was charged with 330 g of water, 91 g of a 33%by weight polymer seed (polystyrene latex, d₅₀ 30 nm), 5.0 g ofterpinolene and 10% of the initiator solution (feed stream 2) and thisinitial charge was heated to 95° C.

Then the remainder of the monomer emulsion and the remainder of theinitiator solution were added to the polymerization vessel by way of twoseparate feeds, beginning simultaneously, over the course of 2.5 hours,during which the temperature was maintained. After the end of theaddition of monomer, the mixture was cooled to 90° C. and then anaqueous solution of 8.5 g of tert-butyl hydroperoxide in 90 g of water,and also a solution of 3.9 g of acetone and 15.7 g of a 40% strength byweight aqueous sodium disulfite solution in 84 g of water were added,beginning simultaneously, over the course of 2 hours, during which thetemperature was maintained. Thereafter, 24.6 g of 25% strength by weightsodium hydroxide solution were added and the batch was cooled to roomtemperature.

Feed Stream 1:

540.0 g deionized water  36.6 g emulsifier solution 950.0 g styrene495.0 g butadiene  45.0 g acrylic acid  12.0 g 25% strength by weightaqueous sodium hydroxide

The feed stream 2 and the emulsifier solution correspond to those inExample 1:

The solids content of the dispersion was about 49.9% by weight. Thelight transmittance was 73.8%. The weight-average particle size d₅₀ was120 nm. The pH was 5.7 and the glass transition temperature T_(g) was27° C.

COMPARATIVE EXAMPLE CE 1

A polymerization was conducted in accordance with Example 2 except thatthe terpinolene was not included in the initial charge but was insteadpresent as an additional component in feed stream 1. All of the otherprocess parameters of Example 3 were retained.

The solids content of the dispersion was about 51.9% by weight. Thelight transmittance was 72.7%. The weight-average particle size d₅₀ was120 nm. The pH was 5.6 and the glass transition temperature T_(g) was26° C.

Table 1: Fractions of volatile organic components in the resultingdispersions before physical deodorization (in ppm, based on the totalweight of the dispersion)

Example 1 2 CE1 Butadiene 50 50 70 VCH 110 110 110 Ethylbenzene 20 20 30Styrene 650 650 2100 PCH 30 50 50 Terpinolene 290 290 480 Σ 1150 11702840

1. A process for preparing an aqueous styrene-butadiene polymerdispersion, comprising: polymerizing, in an aqueous emulsion containinga free radical initiator, by a monomer feed technique, in apolymerization vessel a monomer mixture M comprising from 40 to 80% byweight of styrene as monomer M1, from 20 to 60% by weight of butadieneas monomer M2, and from 0 to 40% by weight, based on 100% by weight ofmonomers, of ethylenically unsaturated comonomers M3 other than styreneand butadiene, the emulsion also containing from 0.05 to 0.5% by weight,based on 100% by weight of monomers, of at least one hydrocarboncompound polymerization regulator selected from the group consisting ofcompounds having from 6 to 20 carbon atoms, which upon abstraction of ahydrogen atom form a pentadienyl or 1-phenylallyl radical, andα-methylstyrene dimer, wherein in said monomer feed technique, at least90% of a total of said monomer mixture M is added to said polymerizationvessel under polymerization conditions, said polymerization vesselhaving also been initially charged with material that contains at least30% of the hydrocarbon compound and then supplying the remainder of thehydrocarbon compound to the polymerization reaction medium in the courseof the polymerization reaction.
 2. The process as claimed in claim 1,wherein the entirety of the hydrocarbon compound is present in theinitial charge to the polymerization vessel.
 3. The process as claimedin claim 1, wherein at a time when at least 70% of the monomers to bepolymerized have been supplied to the polymerization reaction theconcentration of butadiene in the monomer feed is raised for a period ofat least 1% of the total feed duration by at least 10% by weight, basedon monomers in the feed.
 4. The process as claimed in claim 3, whereinthe concentration of the butadiene in the monomer feed is raised in saidperiod to at least 50% by weight.
 5. The process as claimed in claim 3,wherein a monomer mixture comprising styrene, butadiene, and optionalmonomers M3 is supplied to the polymerization reaction as a monomerfeed, and when at least 70% of the monomer feed has been supplied to thepolymerization reaction, from 0.5 to 20% by weight of butadiene, basedon the total amount of butadiene to be polymerized, is supplied to thepolymerization reaction as a second feed in parallel to said monomerfeed.
 6. The process as claimed in claim 5, wherein the total secondfeed is supplied within a time interval which ranges from 1 to 20% ofthe duration of said monomer feed.
 7. The process as claimed in claim 3,wherein the weight fraction of styrene in the monomer feed at the timewhen the supply of monomer to the polymerization reactor is ended isless than 20% by weight.
 8. The process as claimed in claim 7, whereinthe supply of styrene to the polymerization reactor is ended before thesupply of butadiene is ended.
 9. The process as claimed in claim 1,wherein the hydrocarbon compound is selected from the group consistingof terpinolene, γ-terpinene and α-methylstyrene dimer.
 10. The processas claimed in claim 1, wherein the polymerization initiator is selectedfrom the group consisting of organic and inorganic compounds containinga peroxide group.
 11. The process as claimed in claim 1, wherein thepolymerization reaction is conducted in the presence of from 0.1 to 10%by weight of at least one seed latex.
 12. The process as claimed inclaim 1, wherein said optional monomers M3 are selected from the groupconsisting of monoethylenically unsaturated monocarboxylic anddicarboxylic acids having from 3 to 10 carbon atoms, their amides, theirC₂-C₄ hydroxyalkyl esters, their N-(hydroxy-C₁-C₄ alkyl)amides andethylenically unsaturated nitriles.
 13. The process as claimed in claim1, wherein the monomer mixture to be polymerized comprises from 55 to70% by weight of styrene, from 29 to 44% by weight of butadiene, andfrom 1 to 10% by weight of at least one of said ethylenicallyunsaturated comonomers M3.
 14. The process as claimed in claim 5,wherein, when from 75 to 99% of said monomer feed has been supplied tothe polymerization reaction, from 0.5 to 20% by weight of butadiene,based on the total amount of butadiene to be polymerized, is supplied tothe polymerization reaction.
 15. The process of claim 1, wherein thepolymerization is performed in the presence of less than 0.1% by weight,based on monomers to be polymerized, of a polymerization regulator whichis different from the hydrocarbon compound.
 16. The process of claim 1,wherein the material initially charged to the polymerization vesselcontains none of the monomer mixture M, and the entirety of the monomermixture M is added to the polymerization vessel under polymerizationconditions.
 17. The process of claim 1, wherein the material initiallycharged to the polymerization vessel contains none of the monomermixture M, and the entirety of the monomer mixture M is added to thepolymerization vessel under polymerization conditions.
 18. A method,comprising: polymerizing, in an aqueous emulsion containing a freeradical initiator, by a monomer feed technique in a polymerizationvessel, a monomer mixture comprising from 40 to 80% by weight of styreneas monomer M1, from 20 to 60% by weight of butadiene as monomer M2, andfrom 0 to 40% by weight, based on 100% by weight of monomers M3, ofethylenically unsaturated comonomers other than styrene and butadiene,the emulsion also containing from 0.05 to 0.5% by weight, based on 100%by weight of monomers, of at least one hydrocarbon compoundpolymerization regulator selected from the group consisting of compoundshaving from 6 to 20 carbon atoms, which upon abstraction of a hydrogenatom form a pentadienyl or 1-phenylallyl radical and α-methylstyrenedimer, wherein in said monomer feed technique, at least 90% of a totalof said monomer mixture M is added to said polymerization vessel underpolymerization conditions, said polymerization vessel having beeninitially charged with material that contains at least 30% of thehydrocarbon compound and then supplying the remainder of the hydrocarboncompound to the polymerization reaction medium in the course of thepolymerization reaction, thereby preparing an aqueous styrene-butadienepolymer dispersion in which the residual amounts of volatiles in thepolymer dispersion are reduced.
 19. The process of claim 18, wherein thepolymerization is performed in the presence of less than 0.1% by weight,based on monomers to be polymerized, of a polymerization regulator whichis different from the hydrocarbon compound.